Add lowercase proxy vars and make priority consistent with other tools

[jgoodson]

References

Resolves #16286

Code changes

This modifies the code for obtaining a proxy URL from the environment to first check http_proxy/https_proxy, consistent with discussion and recommendations from this Gitlab engineering blog post.

The recommendation from the linked post following a discussion of issues that can arise is:

Prefer lowercase forms over uppercase variables (e.g. http_proxy should be searched before HTTP_PROXY).

This also slightly changes the priority of HTTP vs "ALL" proxy environment variables to favor more specificity. If only HTTP_PROXY is set, behavior will be as before. If both are set, the value of http_proxy will be used.

User-facing changes

This won't have any changed effect if only HTTP_PROXY is set, but it will now detect the more-common http_proxy. If both upper- and lower-case variants are set and have different values (why?) the lower-case variant will be preferred, consistent with other tools. Additionally, if both ALL_PROXY and HTTP_PROXY are set to different values, this code will change the resulting HTTPS proxy to be the URL from HTTP_PROXY and not ALL_PROXY, again consistent with other tools.

Backwards-incompatible changes

No backwards incompatibility, only minor changes in behavior in edge cases. The resulting behavior should be more consistent with that of other tools.

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[krassowski]

LGTM, thank you @jgoodson!

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